1. Field of the Invention
The invention relates to paper machine components and in particular to paper machine dry sections having a plurality of drying cylinders and guide rolls looped about by an endless carrier belt.
2. Description of Related Technology
Dryer sections of paper machines are disclosed, for example, in Kade et al., U.S. Pat. No. 5,050,317 and Preisetanz et al., U.S. Pat. No. 5,177,880. These patents disclose dryer sections with single-row dryer groups in which a plurality of dryer cylinders are disposed in a row and a suction-deflection roll is provided between two neighboring dryer cylinders. An endless carrier or support belt, also known as a dryer wire, loops in alternating fashion around the suction-deflection rolls and the dryer cylinders. During operation, a paper web to be dried is guided by the carrier belt as the paper and the belt travel about a dryer cylinder. The paper web is sandwiched between the surface of the dryer cylinder and the carrier belt.
Normally, a dryer section contains several dryer groups so that a paper web is transferred from one dryer group to the next during its passage through the paper machine.
At an initial region of the dryer section, the paper web has a relatively high water content and thus a relatively low strength. Therefore, the paper web is always supported by the carrier belt of a particular dryer group. The dryer groups are arranged such that the paper web is fully supported at transfer regions between the dryer groups. In such a transfer region, the web comes into contact with a carrier belt of a next dryer group positioned directly downstream with respect to a direction of travel of a paper web through the paper machine, but at the same time remains in contact for a small distance with the carrier belt of the previous (i.e., upstream) dryer group. Thus, the paper web is sandwiched between the two carrier belts for a short time.
However, due to the paper web low strength mentioned above, tearing of the paper web often occurs. Also, large amounts of waste may be produced within a short time due to the high velocity of travel of the paper web through the paper machine. The waste must be removed as fast as possible so that it does not cause damage to the guide rolls, dryer cylinders or dryer screens.
In order to be able to remove paper jam as fast as possible, the transfer region between dryer groups is preferably designed so that it can be "opened" rapidly. This is achieved, for example, by moving a dryer wire of a first dryer group away from a guide roll of a neighboring dryer group via corresponding displacement of a guide roll in the transfer region, so that the waste can fall into a machine cellar. The guide roll can be a suction roll which is located during normal operation within the wire loop of the neighboring dryer group and which is disposed near a last dryer cylinder of the previous (i.e., upstream) dryer group. See, for example, FIG. 1 of U.S. Pat. No. 5,177,880 which shows a suction guide roll near a dryer cylinder 13. FIG. 2 of U.S. Pat. No. 5,050,317 shows a dryer section arrangement in which a guide roll 194 can be displaced so that a dryer wire 174 of a dryer group can be moved away from a dryer wire 175 of the next (i.e. downstream with respect to a direction of travel of a web through the dryer groups) dryer group.
Displacement of guide rolls generally results in a change of the tension of a cooperating dryer wire. To keep the tension constant, tension rolls may be provided within each dryer wire loop. The tension rolls receive signals from tension sensors which first record a change in wire tension and then give an appropriate command to the tension roll. However, as described above, a dryer wire becomes displaced during an "opening" process of a transfer region between dryer groups, as well as during a corresponding closing process of the region, which can lead to significant interruption of the operation of the dryer section and also to continual tearing of the paper web.
In dryer sections known in the art described herein, application of tension to a dryer wire, after it becomes slack, does not take place fast enough. This is due to a certain inertia of the tension roll sensor system and of the tension roll control system.